This invention relates to a method of displaying a color image based on image information in the form of electrical signals and a display apparatus using the method of the invention. This invention uses a display panel having at least two sets of pixels in different colors which constitute a regularly repetitive pattern and applies a coloring substance such as a powder toner to selected areas of the display panel.
For displaying the outputs of computers, facsimiles, scanners, etc., it is prevailing to use electrically driven display apparatus such as CRTs, liquid crystal displays, plasma displays and LED displays. A common advantage of these displays is using no mechanically movable parts. However, these displays are not well suited to the display of a still image for long hours, and these displays become very costly when the screen size is enlarged.
There are some proposals of displaying images on a photosensitive or dielectric sheet in the form of belt by selective application of a coloring substance such as a powder toner to the photosensitive or dielectric sheet. By this method it will be relatively easy to produce wide-screen display apparatus or to display still images. However, there are several problems such as high cost of processing the photosensitive belt, unstable running of the belt, durability of the photosensitive belt, difficulty in displaying color images and darkness of the display screen.
Besides, there are proposals of display apparatus using a magnetic coloring substance in the pixels to display images by selective inversion or migration of the magnetic substance. However, such apparatus are not suited to display of color images and also suffer from darkness of the screen.
JP 59-229578 A shows a different method for displaying color images. The method uses an insulating sheet in the form of belt as the display screen base, and three sets of pixels in three different colors such as red, green and blue are formed on the insulating sheet so as to make a color pattern such as a mosaic pattern or a striped pattern, and each pixel is provided with an electrode. While the display screen is moved, a voltage is selectively applied between the electrodes in the pixels and a number of needle-like electrodes fixed opposite to the display screen in order to electrically charge selected pixels. Then the charged pixels are hidden by a toner, so that the unhidden pixels form a color image.
The method of JP 59-229578 A has several problems. It entails high cost and troublesome operations to form the pixels having electrodes. It is not easy to realize accurate alignment of the pixels in the moving display screen and the fixed needle-like electrodes. The display screen in the form of belt is liable to deteriorate or become dusty by repeated driving. The adhesion of toner to the pixels is unfavorable for color reproducibility and durability of the display screen.
JP 61-290479 A proposes a drastic modification of the method of JP 59-229578 A to omit both the electrodes in the pixels and the fixed needle-like electrodes. According to the proposal, color pixels are formed on a photoconductive layer, and at least two kinds of optical narrow-band filters are provided on the opposite side. The color pixels are selectively charged by scanning with at least two light beams different in wavelength through the filters, and then a conductive toner powder is applied. This method is very complicated and costly and hence will be impracticable.